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Abstract

Nanotwin structures are observed in high-resolution transmission electron microscopy
studies of cubic phase CdS quantum dots in powder form by chemical co-precipitation
method. The deposition of thin films of nanocrystalline CdS is carried out on silicon,
glass, and TEM grids keeping the substrates at room temperature (RT) and 200°C by
pulsed laser ablation. These films are then subjected to thermal annealing at different
temperatures. Glancing angle X-ray diffraction results confirm structural phase transitions
after thermal annealing of films deposited at RT and 200°C. The variation of average
particle size and ratio of intensities in Raman peaks I2LO/I1LO with annealing temperature are studied. It is found that electron-phonon interaction
is a function of temperature and particle size and is independent of the structure.
Besides Raman modes LO, 2LO and 3LO of CdS at approximately 302, 603, and 903 cm−1 respectively, two extra Raman modes at approximately 390 and 690 cm−1 are studied for the first time. The green and orange emissions observed in photoluminescence
are correlated with phase transition.

Keywords:

Background

Colloidal semiconductors' nanocrystals (NCs) have been rigorously studied by various
researchers because their unique physical properties are a function of their particle
size. The bandgap observed in the absorption and emission spectra of semiconductor
quantum dots (QDs) are blueshifted due to confinement of charge carriers. The optical
properties of nanoparticles can also be determined by coupling of confined charges
and confined phonons. The physical properties, especially the light emitting properties,
change drastically as the size of the semiconductor materials become smaller. The
optical and electrical properties of semiconductor NCs for applications in optoelectronic
devices
[1,2] and biological fluorescence labeling
[3,4] are affected by quantum confinement when their typical dimensions are equal to or
smaller than the Bohr radius of exciton. Cadmium sulfide, with a bulk bandgap of approximately
2.42 eV (approximately 512 nm) and exciton Bohr radius of approximately 2.85 nm, is
a candidate for quantum-dot blue light-emitting diodes
[5]. It can also be used in photovoltaic devices. The microstructural characterization
of NCs or QDs with size comparable to Bohr's exciton radius is useful to understand
the light emission mechanism. Recently, our group reported the most common microstructural
defects, i.e., twin structure, stacking faults, and grain boundaries in CdS QDs for
the first time
[6]. The surface and structural defects are expected to have important effects on the
physical properties, particularly the optical properties of the QDs.

Thin films of wide bandgap II-VI semiconductors are of considerable interest as their
emissions cover the technologically attractive blue and green spectral regions. In
particular, CdS thin films attracted more attention because their bandgap emission
is expected to lie very close to the highest sensitivity of the human eye, i.e., green
light. The thin films of CdS quantum dots seek wide applications in photonic devices
like laser, LEDs, etc. Ullrich and his group demonstrated optically pumped laser action
in pulsed laser deposition (PLD)-grown CdS thin films
[7,8]. Artemyev
[9] and Nanda et al.
[10] reported electroluminescence and photocurrent studies in devices fabricated using
CdS nanocrystals. Nizamoglu et al.
[11] fabricated white LEDs using CdS quantum dots hybridized on near-UV LEDs. Various
methods have been used by different researchers to synthesize CdS nanocrystals thin
films
[12-17]. It has been shown that PLD is a versatile technique to maintain stoichiometry of
film because of rapid temperature rise (>1011 K/s) produced by focused pulsed laser beam on the target
[14]. Growth of high quality films at a relatively low substrate temperature by PLD is
possible because high-energy atoms and ions in the laser-induced plasma plume create
a higher surface mobility
[18]. A lot of work has been reported on PLD grown CdS films investigating the effect
of various parameters such as substrates, substrate temperature, laser fluence, laser
wavelength, etc.
[14,19-26]. Still, there is a need of further studies on PLD-grown films for the development
of deeper understanding of their structures for future applications. To the best of
our knowledge, the room temperature (RT) deposition of CdS thin film was never reported
before. The thermal annealing-induced phase transition has been studied in thin films
deposited by different routes
[27-29], but not studied in PLD-grown films.

In this letter, we report on the studies of properties of CdS thin films grown by
PLD, keeping the substrates at RT and 200°C, and the annealing effects on the structural
and optical properties of the films. The variation of average particle size and ratio
of intensities of Raman peaks I2LO/I1LO are studied with respect to the annealing temperature. It seems that electron-phonon
interaction is a function of temperature and particle size, irrespective of the structure.
Two extra modes in Raman spectra have been identified for the first time. These are
verified by low frequency Raman studies.

Methods

Thin films of CdS quantum dots are deposited by laser ablation of a target prepared
by pressing and sintering the chemically synthesized CdS QDs powder. The synthesis
of CdS quantum dots in powder form is reported elsewhere
[6]. For PLD, using ultraviolet laser source, a pulsed excimer KrF laser (Lambda Physik,
Compex Pro 201, Coherent Inc., CA, USA) operating at 248-nm wavelength has been used.
The pulse width of 10 ns and energy of 300 mJ per pulse have been used. The laser
beam with a repetition rate 10 Hz is focused onto a rotating target mounted at an
oblique angle of 30°. The distance between target and substrate is kept as 5.5 cm.
The films are deposited on single-crystal (111) n-type silicon wafers, glass and carbon-coated
Cu grids (for TEM) at two different temperatures: (1) RT and (2) 200°C inside a clean
stainless steel vacuum chamber with a background pressure of 5 × 10−6 mbar. The silicon and glass substrates are cleaned using a standard process that
involves boiling in trichloroethylene followed by rinsing with deionized water. The
CdS deposition rate in this configuration is about 0.025 nm per pulse. The thickness
of deposited film is about 250 nm. To study the post-annealing effect on structural
and optical properties of the films, thermal annealing is carried out at different
temperatures 300°C to 450°C for 3 h in Ar environment.

Results and discussion

CdS quantum dots in powder form are characterized using high resolution transmission
electron microscopy to study the microstructure and defects present in the dots. The
existence of multi-twin structure can be clearly seen in Figure
1a. The twin structures exist with stacking fault and grain boundaries in chemically
synthesized CdS QDs of average particle size of approximately 2.7 nm
[6]. CdS thin films are deposited by PLD using a target prepared by these CdS quantum
dots. TEM micrograph of CdS thin film grown at RT is shown in Figure
1b. It exhibits uniformly distributed CdS nanoparticles of average size of approximately
8.3 nm with a narrow size distribution, as shown in the inset of Figure
1b.

Results of GAXRD scan of films deposited at two different temperatures, (1) RT and
(2) 200°C, and further annealed at different temperatures in Ar environment are given
in Figures
2 and
3, respectively. From Figure
2, it is seen that as-grown film at RT is in mixed phase of cubic and hexagonal structures.
The diffraction peaks at 24.67°, 26.60°, and 28.1° correspond to (100), (002) and
(101) planes of the hexagonal structure. The peaks at 44.15° and 52° are assigned
to (220) and (311) planes of cubic structure. It is observed that after annealing,
diffraction peaks at 24.67° and 28.1° disappear, whereas the intensity of the peak
at 26.60° is enhanced. It is seen from Figure
2 that the mixed phase of as-deposited film transferred to cubic phase after annealing
at 350°C as confirmed by the appearance of diffraction peaks at 26.60°, 43.9°, and
52.25° corresponding to (111), (220), and (311) planes (PCPDF WIN 100454) of the cubic
phase. Further annealing at higher temperature enhances the intensity of peak at 26.6°
corresponding to (111) plane of the cubic phase, and the intensity of other peaks
is reduced. Increase in intensity and sharpness of diffraction peak corresponding
to the plane (111) with increasing annealing temperature shows that the crystallinity
of the film is improved.

Figure 2.GAXRD pattern of CdS thin films deposited at RT and annealed at different temperatures.

Figure
3 shows that as-grown film at substrate temperature of 200°C is also in mixed phase
of cubic and hexagonal structures but with different orientation of planes. The diffraction
peak at 26.64° is assigned to (111) plane of the cubic structure, whereas the peaks
at 36.8° and 48.1° are assigned to (102) and (103) planes of the hexagonal structure,
respectively. It is clearly seen from Figure
3 that comparatively stable hexagonal phase starts to dominate over the mixed phase
at annealing temperature of 350°C. Complete hexagonal phase occurs at 400°C. The diffraction
peaks at 24.8°, 26.48°, 28.12°, 43.64°, 47.8°, and 51.9° correspond to the (100),
(002), (101), (110), (103), and (112) planes (PCPDF WIN 751545) of the hexagonal structure
with lattice parameters a = 4.12 Å and c = 6.724 Å for the film annealed at 350°C. There is one additional diffraction peak
at 31.6° corresponding to (002) plane of the hexagonal phase of Cd. It is also clear
from Figure
3 that there is a phase transition from hexagonal to cubic phase for the film annealed
at 450°C.

Cadmium sulfide has C6V symmetry with four atoms per unit cell. Group theory predicts that there are nine
optical branches at the zone center. These optical branches can be classified as one
symmetric A1 and one doubly degenerate E1 which are both Raman and infrared active, two doubly degenerate E2 branches which are Raman active only, and two antisymmetric with respect to the twofold
and sixfold axes (B1) ‘silent modes’ inactive in both infrared absorption and Raman scattering
[30,31].

In bulk CdS crystals, the phonon eigenstate is a plane wave, and the selection rule
for Raman scattering is q ≈ 0, where q is the wave vector. However, in nanocrystalline materials, q ≈ 0 selection rule is relaxed due to interruption of lattice periodicity. In the
present study, we observe five optical vibrational Raman active modes at approximately
302, 390, 603, 690, and 903 cm−1 in all samples as shown in Figures
4 and
5. The intense and broad peaks at approximately 302, 603, and 903 cm−1 are assigned to fundamental optical phonon mode (LO), the first over tone mode (2LO),
and the second overtone (3LO) of CdS, respectively. These are in agreement with previous
reports
[32]. In nanometer-sized particles, the most prominent peak of CdS may be shifted to 300
cm−1[32]. The reduction in particle size causes noticeable asymmetry and frequency shift towards
the lower frequency side, which has been theoretically calculated and experimentally
observed
[33]. Raman spectra in Figures
4 and
5 show that the first-order LO Raman line is not only broadened but also asymmetric
towards the lower frequency side compared to bulk CdS (305 cm−1). The weak Raman peaks at approximately 390 and 690 cm−1 result from multiphonon scattering. They are identified as those corresponding to
the vibrational modes 1LO + 2E2 and 2LO + 2E2 respectively. The peak at 394 cm−1 is observed previously for CdS microcrystallite-doped glass thin film by Jerominek
et al., but it remained unidentified
[34]. To confirm our observation related to these modes, low frequency Raman spectrum
of as-deposited CdS film deposited at 200°C is shown in Figure
6. Low frequency Raman spectrum shows the presence of E2 branch at approximately 43.3 cm−1 which is reported at 44 cm−1 for bulk CdS in the literature
[31].

It can be clearly seen from Figures
4 and
5 that the peak intensities of 1LO, 2LO, and 3LO modes increase first for films annealed
at 300°C, suddenly decrease for films annealed at 350°C, and then continuously increase
up to highest annealing temperature. According to Sivasubramanian et al., this is
due to the phase transition because change in structure may be the factor, which results
in the change in the electron-phonon coupling
[35]. As the particle size increases with annealing, 2LO line becomes stronger, while
the 1LO line becomes weaker in intensity. The variation of intensity ratio I2LO/I1LO as a function of annealing temperature is shown in Figure
7. The average particle size is calculated for all samples from GAXRD analysis using
Debye Scherrer's formula, and its variation is plotted with annealing temperature
in Figure
7. It can be seen that the behavior of Raman intensities of the fundamental and its
overtone for mixed, cubic, and hexagonal phases are different. However, in either
of the phases, the ratio I2LO/I1LO increases as the temperature/particle size increases. Since the strength of the electron-phonon
interaction is measured by I2LO/I1LO ratio
[35], it may be concluded from the present study that electron-phonon interaction is a
function of temperature and particle size, irrespective of the structure. In bulk
CdS, there is comparatively strong electron-phonon coupling in hexagonal phase than
that in the cubic phase
[36]. It may be concluded that electron-phonon coupling in mixed phase is comparatively
weaker than that in the hexagonal or cubic phase. This is because of the difference
in the symmetries of LO phonons together with nature of the interband transitions
in either of the two phases. Asymmetry of 1LO mode (Figure
4 and Figure
5 inset) varies with electron-phonon interaction, annealing temperature, and particle
size. The intensity of 1LO + 2E2 and 2LO + 2E2 decreases as annealing temperature is increased. This is in agreement with previous
result of Jerominek et al.
[34]. They observed that peak at 394 cm−1 vanished in film annealed at 500°C.

UV-visible absorption spectra are recorded for the films deposited on glass substrate
to study the annealing effect on energy bandgap of CdS thin films. Figures
8 and
9 show the Tauc plots of the films grown at RT and 200°C, and annealed at different
temperatures. The direct bandgap values are calculated as approximately 2.55, 2.39,
and 2.48 eV for as-deposited, annealed at 300°C, and 450°C, respectively, for films
grown at RT by extrapolating the linear fit to the energy axis from the plot of (αhν)2 versus energy. The bandgap of as-grown film at 200°C is estimated as approximately
2.46 eV and becomes minimum for film annealed at 300°C. Further annealing results
in increase in bandgap. It becomes nearly equal to the bandgap of as-deposited film
(approximately 2.44 eV) for the film annealed at 400°C. These results are in agreement
with previous report
[36]. It can be seen from Figure
9 that at 450°C, the bandgap increases suddenly. This may be due to the unexpected
phase transition from hexagonal to cubic structure at 450°C.

Figure 8.Tauc plot of CdS thin films deposited at RT and annealed at different temperatures.

Figure 9.Tauc plot of CdS thin films deposited at 200°C and annealed at different temperatures.

Photoluminescence studies are carried out to get the information regarding different
energy states available between valence and conduction bands, which are responsible
for radiative recombination. PL spectra for the films deposited at 200°C and annealed
at different temperatures are shown in Figure
10. It reveals that as-deposited film shows broad green emission centered at approximately
2.35 eV. The green emission in as-deposited film may be attributed to the transition
of S-vacancy donors to the valence band. It is expected that during deposition at
substrate temperature 200°C, some sulfur vacancies may be created. The intensity of
green emission increases with the increase in annealing temperature and shifts towards
lower energy side from 2.35 to 2.28 eV. It seems that the increase in intensity and
shifting toward lower energy side is due to the increase in sulfur vacancies with
rise in temperature. It can also be seen from Figure
10 that annealing results in an additional broad orange emission peak at approximately
1.87 eV whose intensity increases with rise in temperature except for the film annealed
at 350°C where intensity is decreased drastically. The origin of the orange emission
may be due to the transition between interstitial cadmium donor level and acceptor
level located at higher energies with respect to the valence band
[36]. The decrease in intensity of this peak at 350°C may be due to the structural phase
transition.

Conclusions

The influence of annealing in Ar environment on PLD-grown CdS thin films at two different
temperatures, (1) RT and (2) 200°C has been studied. The annealing-induced phase transition
is observed in both types of films with different features. The structural phase transition
is correlated with variation in particle size, bandgap, intensity ratio of 2LO to
1LO (I2LO/I1LO) Raman peaks, asymmetry in 1LO mode, and PL results. It may be concluded that in
either of the phases viz. mixed, cubic or hexagonal, the ratio I2LO/I1LO increases as the temperature and particle size increase, and electron-phonon interaction
is a function of temperature and particle size, irrespective of the structure. The
existence of the two extra Raman active modes at approximately 390 and 690 cm−1 is observed using low frequency Raman measurement.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

PK conceived the idea. PK and NS performed the experiments. AA and DK supervised the
project. RC and VG provided the facilities and discussions related to them. PK and
NS co-wrote the paper. All the authors read and approved the final manuscript.

Authors’ information

PK is a research scholar under the supervision of AA, an assistant professor at the
Department of Physics, Bareilly College, Bareilly, India. NS is research associate
working with DK at Inter University Accelerator Centre, New Delhi, India. RC and VG
are professors at Indian Institute of Technology, Roorkee and University of Delhi,
respectively.

Acknowledgments

The authors are thankful to Dr. RP Singh, the principal of Bareilly College, Bareilly
for providing the necessary facilities and moral support. The assistance provided
by Mr. Ravish Jain and Mr. Paritosh Dubey, Indian Institute of Technology, Roorkee
during thin film deposition is highly appreciated. The help received from Mr. Pawan
Kulariya and Dr. Vinod Kumar, IUAC, New Delhi in GAXRD measurements and Raman measurements
is gratefully acknowledged. The authors are also thankful to Ms. Kajal Jindal from
the University of Delhi for the PL and low frequency Raman measurements.